Why the Future Belongs to Jellyfish

Jellyfish: they’re the worst. From the dollar bill-sized jellies that wash up along the New Jersey shore to the deadly box jelly—the most toxic creature on the planet—no one likes jellyfish. And the bad news is that they may be taking over: as we pull fish from the sea, the jellyfish are left to flourish. Overfishing, climate change and pollution are also setting the stage for the rise of the jellyfish. It could almost be a case of evolution running backwards—complex, vertebrate biodiversity replaced by a monoculture of squishy things, thanks chiefly to us. We could end up with what scientists call a “gelatinous ocean” dominated by jellyfish—which is exactly as appealing as it sounds like.

But hold on a minute. In the competition for food and other resources, vertebrate fish should have the upper hand—they have eyes, for one thing, and they can directly chase prey. Jellyfish essentially float around, pulsing water through their bodies to sweep up food. In scientific terms, jellyfish should have lower “clearance rates“—the amount of water they can strip of prey per unit of time. That should make them less competitive.

But a new study in the September 15 Science argues that jellyfish may be better predators than we think. Researchers led by Jose Luis Acana of the Universidad de Oviedo in Spain compared the feeding strategies and biology of jellyfish to vertebrate fish. They found that the jellies are plenty efficient hunters and feeders. Over hundreds of millions of years, jellyfish have evolved huge, water-laden bodies—a jellyfish might contain less than 1.5 kg of carbon per cubic meter of body weight, compared to 100 kg of carbon for solid organisms. That massive size—the Lion’s Mane jelly has tentacles as long as 120 ft.—enables jellyfish to come into contact more frequently with prey. The fact that jellyfish float rather than actively swim saves them considerable energy as well—and that energy can go to building those big, puffy bodies.

From the Science study:

It seems that jellyfish have responded to the selective pressure of prey dilution typical of life as predators by evolving large bodies and collection structures while keeping the dynamic component of the feeding process, the swimming velocity, slow. Unlike fish, being large and slow may render jellyfishes more vulnerable to predation and passively dependent on ocean currents for sex encounter and recruitment to benthos. However, our work suggests that their bioenergetic performance is very similar. By using their primitive feeding systems, jellyfishes achieve instantaneous production rates similar to those of fishes and are capable of capitalizing on ecosystem changes resulting from overfishing.

In other words, jellyfish can already hold their own against vertebrate fish in the normal ocean environment. Should we remove those fish—which we’re already on the way to doing—jellyfish should have no trouble moving right into the neighborhood. That has economic costs—jellyfish invasions already frequently force the closure of popular beaches. But the idea of a gelatinous ocean is also—with just a bit of vertebrate bias on my part—totally gross.